This invention relates to diamond and more particularly to diamond produced by chemical vapour deposition (hereinafter referred to as CVD).
Methods of depositing materials such as diamond on a substrate by CVD are now well established and have been described extensively in the patent and other literature. Where diamond is being deposited on a substrate, the method generally involves providing a gas mixture which, on dissociation, can provide hydrogen or a halogen (e.g. F, Cl) in atomic form and C or carbon-containing radicals and other reactive species, e.g. CHx, CFx wherein x can be 1 to 4. In addition, oxygen containing sources may be present, as may sources for nitrogen and for boron. In many processes inert gases such as helium, neon or argon are also present. Thus, a typical source gas mixture will contain hydrocarbons CxHy wherein x and y can each be 1 to 10 or halocarbons CxHyHalz (Hal=halogen) wherein x and z can each be 1 to 10 and y can be 0 to 10 and optionally one or more of the following: COX, wherein x can be 0.5 to 2, O2, H2, N2, NH3, B2H6 and an inert gas. Each gas may be present in its natural isotopic ratio, or the relative isotopic ratios may be artificially controlled; for example hydrogen may be present as deuterium or tritium, and carbon may be present as 12C or 13C. Dissociation of the source gas mixture is brought about by an energy source such as microwaves, RF energy, a flame, a hot filament or a jet based technique and the reactive gas species so produced are allowed to deposit onto a substrate and form diamond.
CVD diamond may be produced on a variety of substrates. Depending on the nature of the substrate and details of the process chemistry, polycrystalline or single crystal CVD diamond may be produced. The production of homoepitaxial CVD diamond layers has been reported in the literature.
European Patent Publication No. 0 582 397 describes a method of producing a polycrystalline CVD diamond film having an average grain size of at least 7 microns and a resistivity, carrier mobility and carrier lifetime yielding a collection distance of at least 10 μm at an electric field strength of 10 kV/cm. This is a diamond film of a quality which makes it suitable for use as a radiation detector. However, applications for films having collection distances as low as 7 μm are very limited.
European Patent Publication No. 0 635 584 describes a method of producing a CVD polycrystalline diamond film using an arc jet process with low methane levels (less than 0.07%) and an oxidant. The diamond material has a narrow Raman peak, a relatively large lattice constant, and a charge carrier collection distance of greater than 25 μm. However, the performance of polycrystalline diamond films in electronic applications is believed to be adversely affected by the presence of grain boundaries.
It has not previously been reported that single crystal CVD diamond can be grown with sufficient control to achieve high performance detector material. Collection distances measured on natural single crystal diamond have been reported of about 28 μm at 10 kV/cm and 60 μm at bias voltages of 26 kV/cm. In high quality type IIa natural single crystal diamond the collection distance has been shown to vary nearly linearly with bias voltage up to 25 kV/cm, unlike polycrystalline material which typically shows saturation of the collection distance at about 10 kV/cm.
The collection distance can be adversely affected in a single crystal diamond by the presence of impurities and lattice defects which reduce the free carrier mobility and free carrier recombination lifetime of the carrier.
Prior art has generally concerned itself with the thermal, optical and mechanical properties of CVD diamond.